(1) Field of the Invention
This invention relates to a method of removing carbon dioxide by adsorptive separation from a gaseous mixture containing carbon dioxide and gases less polar than carbon dioxide.
(2) Description of the Related Art
Carbon dioxide is contained in natural gas, exhaust gas from combustion, as well as atmospheric air although in a minor amount. Further, carbon dioxide is produced as by-product in industrial processes, for example, at a step of steam-reforming natural gas, naphtha, coke or methanol to produce hydrogen. In recent years carbon dioxide industrially produced as by-product including that contained in combustion gas has been attracting widespread attention because it causes global warming. Further, cryogenic separation of air has a problem such that a trace amount, i.e., about 300 to 400 ppm, of carbon dioxide in air is solidified upon cooling, leading to clogging an equipment such as a heat exchanger.
As the method of separating and removing carbon dioxide contained in a gas, there can be mentioned a method of chemically absorbing carbon dioxide in a solution of an alkali or amine, and a method of physically adsorbing carbon dioxide by an adsorbent such as active carbon or a zeolite. The methods of physical adsorption of carbon dioxide using a zeolite adsorbent include a temperature/pressure swing adsorption (PTSA) method and a pressure swing adsorption (PSA) method. In these methods, adsorption of carbon dioxide by a zeolite adsorbent is effected at a low temperature and a high pressure and desorption thereof from the zeolite adsorbent for the reproduction of the zeolite adsorbent is effected at a temperature higher and a pressure lower than those for adsorption. Upon desorption, the zeolite adsorbent may be purged with a gas which contains no carbon dioxide and is less adsorbed than carbon dioxide.
It is known that a zeolite adsorbent adsorbs a molecule by the interaction between the cation present in the zeolite adsorbent and the molecule. The interaction is enhanced and the amount of the molecule adsorbed increases with an increase of polarity of the molecule. For example, the descending order of interaction between the zeolite and ingredients in air is water, carbon dioxide, nitrogen, oxygen and then argon. The descending order of the amount of adsorption is also the same.
Assuming that carbon dioxide in atmospheric air is removed by adsorption, the content of carbon dioxide in air is about 300 ppm and the content of nitrogen in air is about 78%, and hence, when air is brought into contact with an adsorbent at a pressure of 5 to 10 atm., the partial pressure of carbon dioxide is about 2 mmHg and the partial pressure of nitrogen is about 4 to 8 atm. Carbon dioxide has a large polarity and, even when the partial pressure thereof is low, it should be adsorbed in a large amount. But, the amount of carbon dioxide adsorbed is small because its adsorption is hindered by the presence of a large amount of nitrogen. Note, water in air can be substantially completely removed by previously treating with an adsorbent such as alumina.
The zeolite adsorbent hitherto used for removing carbon dioxide from a gaseous mixture such as air includes a type A zeolite and a type X zeolite having an SiO2/Al2O3 molar ratio of at least 2.5. When air is subjected to cryogenic separation by using these zeolite adsorbents, a large amount of the zeolite adsorbents must be used because the amount of air treated for cryogenic separation is very large. Therefore, to reduce the size of equipment or reduce the energy consumption, an adsorbent exhibiting a high adsorption for carbon dioxide even in the co-presence of a large amount of nitrogen is eagerly desired.
A method of removing carbon dioxide from a gas stream by using a zeolite adsorbent has been proposed in Japanese Unexamined Patent Publication (abbreviated to xe2x80x9cJP-Axe2x80x9d) No. H8-252419 (corresponding to U.S. Pat. No. 5,531,808) wherein the gas stream is contacted with a type X zeolite having a silicon/aluminum atomic-ratio of about 1.0 to about 1.15 and having been ion-exchanged with a cation selected from the ions of group 1A, group 2A, group 3A, group 3B, the lanthanide series and mixtures thereof at a temperature of about xe2x88x9250xc2x0 C. to about +80xc2x0 C. It is noted that the change of the uptake of carbon dioxide depending upon the pressure of carbon dioxide is examined (see table on page 6 of JP-A ""419 [in col. 6 of U.S. ""808]), but, the selective adsorption of carbon dioxide in a gaseous mixture containing carbon dioxide and nitrogen is not examined therein.
Further, as preferable exchangeable cations, sodium and lithium falling in the ions of group 1A and calcium falling in the ions of group 2A are mentioned in JP-A ""419 (U.S. ""808) More specifically a sodium-exchanged type X zeolite (NaLSX) and a lithium- and calcium-exchanged type X zeolite (Li,CaLSX) (the amounts of lithium and calcium are 95 equiv. % and 5 equiv. %, respectively) are examined, and it is shown that Li,CaLSX is superior to NaLSX in the uptake of carbon dioxide, but, the adsorption selectivity between carbon dioxide and nitrogen is not examined.
A method of removing water vapor and carbon dioxide from a gas wherein water vapor is first removed and then carbon dioxide is removed by using sodium LSX zeolite as adsorbent has been proposed in JP-A H8-179,137 (corresponding to U.S. Pat. No. 5,914,455). However, this patent is silent on the removal of carbon dioxide from a gaseous mixture containing carbon dioxide and nitrogen.
A method of removing carbon dioxide from a gas such as atmospheric air wherein carbon dioxide is adsorbed by type X zeolite having an Si/Al atomic ratio of 1 to 1.5 and having been ion-exchanged with calcium, sodium and potassium has been proposed in JP-A H11-253,736.
A zeolite adsorbent for gas purification comprising a sodium-type low-silica faujasite having an SiO2/Al2O3 of about 1.8 to 2.2 with a residual content of potassium ions less than about 8.0 equiv. % and a binder has been proposed in WO 00/01478. The zeolite adsorbent used therein has a low zeolite crystal purity and contains a binder, and hence, the adsorption performance inherently possessed by zeolite crystal is not sufficiently manifested.
In general zeolite adsorbents are used in the form of a shaped product prepared by incorporating a binder in zeolite and shaping the mixture of zeolite and the binder into a desired shape. However, a binder has no adsorption performance, and hence, the zeolite adsorbent containing a binder has a relatively poor adsorption performance. Therefore, proposals of using a binder capable of being converted to zeolite have been made. For example, a method of producing a low-silica type X zeolite binderless shaped product has been proposed in JP-A H5-163,015 wherein a shaped product comprised of a type X zeolite powder having an SiO2/Al2O3 molar ratio of smaller than 2.5, metakaolin converted from kaolin clay, sodium hydroxide and potassium hydroxide is maintained in an aqueous solution containing sodium hydroxide and potassium hydroxide at a temperature of 40 to 100xc2x0 C. for several hours to several days whereby metakaolin is converted to zeolite to give a low-silica type X zeolite binderless shaped product.
A shaped product comprised of at least 95% of a low-silica type X zeolite having an SiO2/Al2O3 molar ratio of at least 2, which is prepared by using a binder capable of being converted to a zeolite, is described in JP-A H11-76810. This shaped product is prepared by a process wherein a low-silica type X zeolite is agglomerated by using a binder containing at least 80% of a clay capable of being converted to a zeolite; the thus-obtained agglomerate is shaped; the shaped product is dried and then calcined at a temperature of 500 to 700xc2x0 C.; and the thus-obtained solid product is placed in contact with an aqueous alkali solution containing an alkali at least 0.5 molar concentration comprising sodium hydroxide and potassium hydroxide, wherein the proportion of potassium hydroxide is not larger than 30% by mole based on the sum of sodium hydroxide and potassium hydroxide. The low-silica type X zeolite shaped product has very low crush strength and contains a small amount of type A zeolite. Thus, the SiO2/Al2O3 molar ratio as determined by chemical analysis is larger than the theoretical value, i.e., 2.0, and the low-silica type X zeolite in the shaped product has a low purity.
Further a shaped product comprised of at least 95% of a low-silica type X zeolite having an SiO2/Al2O3 molar ratio of at least 2, which is prepared by using a binder capable of being converted to a zeolite, is described in WO 99/05063. This shaped product is prepared by a process which is similar to the above-mentioned process described in JP-A H11-76810 and in which a shaped product of an agglomerate comprised of LSX zeolite and a clay capable of being converted to zeolite is placed in contact with an aqueous alkali solution containing an alkali at least 0.5 molar concentration. This shaped product of a low-silica type X zeolite has a low crush strength, and this literature is silent on the use thereof for adsorptive separation of carbon dioxide, especially carbon dioxide from air, and suggests nothing about the adsorptive selectivity between carbon dioxide and nitrogen.
A primary object of the present invention is to provide an improved method of adsorptive separation of carbon dioxide from a gaseous mixture comprising carbon dioxide and gases less polar than carbon dioxide, for example, adsorptive separation of carbon dioxide which is a cumbersome ingredient in air when air is subjected to cryogenic separation. More particularly, it is intended by the present invention to effect the adsorptive separation of carbon dioxide by using a binderless zeolite adsorbent, which has a high strength withstanding compressive force imposed in the industrial use and which exhibits an enhanced adsorption capacity. When a zeolite ion-exchanged with a specific alkali metal cation such as sodium ion to a high degree is used, the adsorption selectivity of carbon dioxide and the efficiency of adsorptive separation thereof can be more enhanced.
In accordance with the present invention, there is provided a method of adsorptive separation of carbon dioxide from a gaseous mixture comprising carbon dioxide and gases less polar than carbon dioxide comprising contacting the gaseous mixture with a zeolite adsorbent whereby carbon dioxide is adsorbed by the zeolite to be thereby separated, characterized in that carbon dioxide present in the gaseous mixture as contacted with the zeolite adsorbent has a partial pressure in the range of 0.1 to 50 mmHg, and the zeolite adsorbent is a shaped product comprised of at least 95%, as determined on the basis of the moisture equilibrium adsorption value, of a low-silica type X zeolite having an SiO2/Al2O3 molar ratio in the range of 1.9 to 2.1.
The shaped product preferably has been ion-exchanged with at least one kind of cation selected from lithium and sodium, more preferably with sodium, at an ion exchange ratio of at least 90%, more preferably at least 95%, and further preferably has an average crush strength of at least 1.0 kgf as measured on particles of the shaped product having a particle diameter in the range of 1.4 to 1.7 mm as prepared for crush strength measurement.
The shaped product is preferably comprised of at least 98%, as determined on the basis of the moisture equilibrium adsorption value, of the low-silica type X zeolite. More preferably the shaped product consists essentially of the low-silica type X zeolite and is substantially free from a binder.
The shaped product preferably has a macro pore volume of at least 0.25 ml/g and an average pore diameter of 0.35 xcexcm, and it is preferably a particle having an average particle diameter in the range of 0.5 to 3 mm, more preferably of 0.5 to 2 mm.
The adsorptive separation of carbon dioxide may be effected by pressure swing adsorption (PSA) and/or temperature swing adsorption (TSA). Preferably the adsorption is carried out at a temperature of 0 to 70xc2x0 C., and then desorption is carried out at a temperature of 40 to 200xc2x0 C.
The shaped product can be prepared by a process comprising the steps of (i) shaping a mixture comprising a low-silica type X zeolite having an SiO2/Al2O3 molar ratio in the range of 1.9 to 2.1 and a kaolin clay having an SiO2/Al2O3 molar ratio in the range of 1.9 to 2.1, (ii) calcining the thus-shaped product, and then (iii) placing the calcined shaped product in contact with a caustic solution capable of dissolving silicon from the calcined shaped product in an amount larger than that of aluminum dissolved from the calcined shaped product whereby the kaolin clay contained in the calcined shaped product is converted to a low-silica type X zeolite. Preferably the calcined shaped product is kept in contact with a caustic solution of at least 6 moles per liter, for at least 10 hours, or a caustic solution of at least 8 moles per liter, for at least 5 hours. The caustic solution with which the calcined shaped product is placed in contact may contain an aluminum ingredient previously incorporated therein.
The adsorbent used in the present invention is a shaped product comprised of at least 95%, preferably at least 98%, as determined on the basis of the moisture equilibrium adsorption value, of a low-silica type X zeolite having an SiO2/Al2O3 molar ratio in the range of 1.9 to 2.1. By the phrase xe2x80x9cas determined on the basis of the moisture equilibrium adsorption valuexe2x80x9d used herein, we mean that the content in % of the low-silica type X zeolite in the shaped product is defined by the following equation:
A=[B/C]xc3x97100
where
A: content (%) of low-silica type X zeolite in shaped product,
B: moisture equilibrium adsorption value of the shaped product, and
C: moisture equilibrium adsorption of the low-silica type X zeolite having the same weight as the shaped product.
Most preferably the shaped product consists essentially of low-silica type X zeolite and is substantially free from a binder. The shaped product preferably has been ion-exchanged with at least one kind of cation selected from lithium and sodium, more preferably with sodium, at an ion exchange ratio of at least 90%, more preferably at least 95%.
As explained above, the adsorption of gas by a zeolite occurs due to the interaction between cations present in the zeolite and gas molecules, and thus, with an increase in number of cations present in the zeolite, the capacity of gas adsorption increases. The number of cations present in a zeolite molecule varies depending upon the number of aluminum in the crystal skeleton structure of zeolite, namely, as the number of aluminum increases (i.e., the SiO2/Al2O3 molar ratio of the zeolite is reduced), the number of cations increases. The minimum value of SiO2/Al2O3 molar ratio of zeolite is known as 2.0 by the Loewenstein theory. The low-silica type X zeolite used in the present invention has the minimum SiO2/Al2O3 molar ratio, but, in consideration of measurement inaccuracy of chemical analysis and other factors, the zeolite used is defined as having an SiO2/Al2O3 molar ratio in the range of 1.9 to 2.1. A type A zeolite also is known as having an SiO2/Al2O3 molar ratio of 2.0, but its pore diameter is small, i.e., about 4 to 5 angstroms, and thus, is not advantageous for separation of gas. In contrast, the low-silica type X zeolite has a pore diameter of about 7 to 8 angstroms, and is suitable for gas separation.
The low-silica type X zeolite crystal can be synthesized by various methods. For example, methods described in JP-A H11-217212, JP-A H10-310422 and JP-A H11-343112 can be adopted.
A zeolite crystal powder has no self-binding property and hence a binder is incorporated therein to be shaped into beads, pellets or other forms for an industrial use as an adsorbent. A binder usually has no capability of adsorbing gases, and therefore, the adsorption capacity of commercially available adsorbents is poor as compared with the adsorption capacity of a zeolite crystal powder. Therefore, a binder capable of being converted to a zeolite crystal is used in the present invention to obtain a shaped product comprised of a low-silica type X zeolite having a good adsorption capacity and substantially free from a binder.
The shaped product of a low-silica type X zeolite used in the present invention can be prepared by a process described below. As a low-silica type X zeolite powder used as a raw material, those which are prepared by methods described, for example, in JP-A H11-217212, JP-A H10-310422 and JP-A H11-343112 can be mentioned. To 100 parts by weight of this low-silica type X zeolite powder, 10 to 50 parts by weight of a kaolin clay binder capable of being converted into a low-silica type X zeolite was incorporated together with an appropriate amount of water, and the resulting mixture is uniformly kneaded together. When the amount of kaolin clay is too small, the resulting shaped product does not have a high crush strength. In contrast, when the amount of kaolin clay is too large, crystallization does not proceed to the desired extent, and the low-silica type X zeolite in the resulting shaped product has a poor purity. The amount of water incorporated for kneading varies depending upon the amount of kaolin clay and a manner in which the kneaded mixture is shaped into pellets or beads. Various organic and inorganic shaping aids may be added for enhancing the shapability provided that the subsequent calcination and conversion of kaolin clay into a zeolite are not badly influenced to any appreciable extent. The mixture can be shaped into pellets, beads or other forms by an appropriate method. For example, an extrusion-pelletizing method for pellets, and a stirring or rolling method for beads can be employed.
The pellets, beads or other shaped products are dried and then calcined at a temperature of 500 to 700xc2x0 C., preferably 600 to 650xc2x0 C. to obtain a shaped product containing low-silica type X zeolite. The calcination is essential for enhancing the convertibility of kaolin clay into a low-silica type X zeolite. That is, kaolin clay is changed to amorphous metakaolin by calcination, which is relatively easily converted into a low-silica type X zeolite. The drying and calcination can be carried out by ordinary means using, for example, a hot-air dryer, a muffle furnace, a rotary kiln and a tubular furnace.
The calcined shaped product of a low-silica type X zeolite is then placed in contact with a caustic solution capable of dissolving silicon from the calcined shaped product in an amount larger than that of aluminum dissolved from the calcined shaped product whereby the kaolin clay contained in the calcined shaped product is converted to a low-silica type X zeolite (said treatment of contacting the calcined shaped product with the caustic solution to convert the kaolin clay binder to a low-silica type X zeolite and thus to reduce the content of binder is hereinafter abbreviated to xe2x80x9cbinderless treatmentxe2x80x9d when appropriate). The caustic solution capable of dissolving silicon from the calcined shaped product in an amount larger than that of aluminum dissolved from the calcined shaped product is, for example, a caustic solution exhibiting a higher solubility for a silicate than that for an aluminate. The solubility of a solution varies depending upon the composition and concentration thereof, and temperature, and hence, the composition and concentration of the caustic solution varies depending upon the temperature at which the contacting is effected.
The caustic solution used is preferably a solution containing sodium hydroxide and potassium hydroxide. The proportion of sodium hydroxide and potassium hydroxide is preferably such that the atomic ratio of K/(Na+K) is in the range of 0.1 to 0.4. When the atomic ratio is smaller than 0.1 or larger than 0.4, metakaolin in the shaped product is difficult to be converted into a low-silica type X zeolite to the desired extent and impurities such as type A zeolite, sodalite, type F zeolite and type E zeolite tend to be produced, and thus, a shaped product containing a low-silica type X zeolite at a desired high concentration is difficult to obtain.
The amount of an alkali metal hydroxide in the caustic solution used for the binderless treatment must be at least about 5 times of the amount required for converting the entire amount of kaolin clay contained in the low-silica type X zeolite-containing shaped product into a low-silica type X zeolite. To obtain a low-silica type X zeolite-containing shaped product with a high purity within a short period of treating time, the amount of an alkali metal hydroxide in the caustic solution is preferably at least about 10 times of the amount required for converting the entire amount of kaolin clay into a low-silica type X zeolite. However, if the amount of an alkali metal hydroxide in the caustic solution is too large, i.e., at least 30 times or larger, the production cost increases and the amounts of aluminum and silicon dissolved from the shaped product become undesirably large, which leads to reduction of crush strength of the shaped product.
In the case where a caustic solution containing sodium hydroxide and potassium hydroxide at the atomic ratio described above is used, the concentration of the sum of sodium hydroxide and potassium hydroxide in the solution is at least about 6 moles/liter and preferably at least about 8 moles/liter, for the dissolution of silicon from the calcined shaped product in an amount larger than that of aluminum dissolved from the calcined shaped product. With an increase of the caustic concentration, the amount of a silicate relative to that of an aluminate in the caustic solution is increased and the efficiency of the contact treatment therewith is enhanced. Even when the caustic concentration is about 6 moles/liter, if the time of the contact treatment is too short, metakaolin in the shaped product is difficult to be converted into a low-silica type X zeolite to the desired extent.
An aluminum ingredient can be previously incorporated in the caustic solution used for the binderless treatment. The aluminum ingredient includes soluble and insoluble ingredients, and it is not particularly limited provided that aluminum can be positively used for the formation of a low-silica type X zeolite from metakaolin. As specific examples of the aluminum ingredient, there can be mentioned sodium aluminate, and a low-silica type X zeolite, kaolin clay or other aluminum-containing solid. It is also preferable that the caustic solution used for binderless treatment and containing a residual aluminum ingredient is repeatedly used. In the case where a caustic solution having incorporated therein an aluminum ingredient is used, even when the caustic concentration is lower than that of a caustic solution containing no aluminum ingredient, the conversion of metakaolin to a low-silica type X zeolite to the desired extent can be achieved.
The binderless treatment of the shaped product with the caustic solution is carried out at a temperature of at least 40xc2x0 C. The efficiency of the binderless treatment increases with an elevation of the temperature, but, a preferable treating temperature is in the range of 70 to 80xc2x0 C. in view of the fact that the conversion of metakaolin to a low-silica type X zeolite is an exothermic reaction, and further in view of the limitation of heat-resisting temperature of equipment material, and the minimization of impurity production.
With an increase of the caustic concentration, the time of binderless treatment can be short. When the caustic concentration is at least 6 moles/liter and at least 8 moles/liter, the time of binderless treatment required for the conversion of metakaolin to a low-silica type X zeolite to the desired extent is usually at least 10 hours and at least 5 hours, respectively.
The manner in which the low-silica type X zeolite-containing shaped product is contacted with the caustic solution is not particularly limited. It is preferable in view of ease and efficiency that the caustic solution is passed through a fixed bed column packed with the low-silica type X zeolite-containing shaped product.
The kind of cation of a zeolite adsorbent is an important factor for the adsorptive separation of carbon dioxide. To enhance the selectivity of adsorptive separation of carbon dioxide, the caustic-treated low-silica type X zeolite-containing shaped product is preferably ion-exchanged with at least one kind of cation selected from lithium and sodium. more preferably with sodium, at an ion exchange ratio of at least 90% more preferably at least 95%. The highest adsorption selectivity of carbon dioxide is obtained by an ion-exchange with sodium at an ion exchange ratio of at least 95%.
For example, when air is subjected to cryogenic separation at a low temperature, it is preferable that the air is pre-treated with a zeolite adsorbent for the removal by adsorptive separation of carbon dioxide. In the case where the zeolite adsorbent is placed in contact with air, the pressure of air is generally in the range of about 5 to 10 atm. Atmospheric air usually contains about 300 ppm of carbon dioxide and hence the pressure of carbon dioxide is usually about 1 to 2.5 mmHg. Whereas, the air contains about 78% of nitrogen and hence the pressure of nitrogen is in the range of about 4 to 8 atm. Therefore, although the interaction between the zeolite and carbon dioxide is larger than that between the zeolite and nitrogen, the amount of adsorption of carbon dioxide is greatly influenced by nitrogen. Thus, the zeolite adsorbent used must exhibit an enhanced adsorption selectivity for carbon dioxide even when a salient amount of nitrogen is co-present. The low-silica type X zeolite-containing shaped product as ion-exchanged with an alkali according to the present invention satisfies this requirement.
The method of ion-exchanging the low-silica type X zeolite-containing shaped product with lithium and/or sodium can be conventional, and includes, for example, a batchwise method and a continuous method wherein a lithium- and/or sodium-containing solution is passed through a column packed with the low-silica type X zeolite-containing shaped product. The conditions under which the ion-exchange is effected are not particularly limited. But, a high temperature, especially 50xc2x0 C. or higher, is preferable in view of the ion-exchange equilibrium and the rate of reaction. A high alkali metal hydroxide concentration, usually an alkali metal hydroxide concentration of at least 1 mole/liter, is preferable.
The lithium- and/or sodium-containing compounds used for ion-exchange are not particularly limited provided that they are soluble in water, and the compounds include, for example, chloride, nitrate, sulfate and carbonate. Lithium is expensive as compared with sodium, and hence, a lithium-containing solution can be repeatedly used by removing impurities from the solution as used for ion exchange.
The high-purity low-silica type X zeolite-containing shaped product before the ion exchange according to the present invention usually contains sodium and potassium an ion-exchangeable cations. Therefore, when the shaped product is ion-exchanged with lithium and/or sodium, it is possible that certain amounts of originally contained sodium and potassium remain in the ion-exchanged shaped product. Although the ion-exchanged shaped product contains the residual sodium and potassium, it can be used as an adsorbent according to the present invention. The ion-exchanged shaped product may contain, in addition to lithium and/or sodium, other cations such as those of calcium, magnesium or other divalent metals, and those of zinc and other transition metals.
The as-ion-exchanged shaped product is dried to some extent and then calcined at a temperature of 500 to 550xc2x0 C. in dry air or nitrogen atmosphere to give an adsorbent.
The high-purity low-silica type X zeolite-containing shaped product prepared by the above-mentioned process has an extremely enhanced crush strength, namely, an average crush strength of at least 1.0 kgf as measured when the shaped product is classified into particles with a particle diameter in the range of 1.4 to 1.7 mm. Thus, the shaped product can be advantageously used under industrial conditions.
It is preferable that the high-purity low-silica type X zeolite-containing shaped product exhibits enhanced rates of adsorption and desorption for carbon dioxide, in addition to a high equilibrium adsorption of carbon dioxide. For example, in the case where the adsorptive separation of carbon dioxide from atmospheric air is effected by using a zeolite adsorbent, the rates of adsorption and desorption are influenced by a macro-pore structure of the zeolite adsorbent. The macro-pores are formed among the zeolite crystal particles, and the mass transfer rate is controlled by the macro-pores. Therefore, a large pore volume and a large pore diameter are preferable. The high-purity low-silica type X zeolite-containing shaped product prepared by the above-mentioned process has a macro pore volume of at least 0.25 ml/g and an average pore diameter of at least 0.35 xcexcm. Therefore, there can be obtained a mass transfer rate at which the adsorption capacity inherently possessed by the adsorbent is not reduced. If the macro pore volume and the average pore diameter are smaller than the above values, the mass transfer rate is not sufficiently high and the adsorption capacity inherently possessed by the adsorbent cannot be manifested.
The shaped product used in the present invention can be in the form of pellets or beads, and preferably has an average particle diameter in the range of 0.5 to 3 mm, more preferably 0.5 to 2 mm. At an average particle diameter of larger than 3 mm, the surface area per unit volume of the adsorbent-packed bed is small, and the mass transfer rate within each particle is reduced to an undesirable extent. At an average particle diameter of smaller than 0.5 mm, the pressure drop of the adsorbent-packed bed is too large and the energy consumption is undesirably increased. The average particle diameter can be determined by an ordinary method which includes, for example, a sieving method and a method by calculating from the pressure loss when a gas stream is passed through the adsorbent-packed bed according to the Ergun equation.
The adsorptive separation of carbon dioxide using the above-mentioned low-silica type X zeolite-containing adsorbent according to the present invention can be effected by a pressure swing adsorption method wherein the pressure is repeatedly increased and reduced, or a temperature swing adsorption method wherein the temperature is repeatedly elevated and lowered, or a combination of the pressure swing adsorption method with the temperature swing adsorption method.
The adsorption and desorption of carbon dioxide are preferably carried out at a temperature in the range of 0 to 70xc2x0 C. and 40 to 200xc2x0 C., respectively.
Thus, when the adsorptive separation of carbon dioxide is conducted by using the above-mentioned high-purity low-silica type X zeolite adsorbent according to the method of the present invention, a high adsorption selectivity is obtained, a mall-size separation apparatus can be employed, and the energy consumption is reduced.
In the method of the present invention, carbon dioxide is separated from a gaseous mixture comprising carbon dioxide and gases less polar than carbon dioxide. Carbon dioxide has a strong polarity, and thus, most gases other than carbon dioxide are less polar than carbon dioxide, which include, for example, hydrogen, oxygen, nitrogen and argon. Hence, the method of the present invention can be employed, for example, for purification of air when cryogenic separation of air is conducted, or for purification of natural gas.
The adsorptive separation of carbon dioxide can be advantageously effected when carbon dioxide present in a gaseous mixture such as air or natural gas has a partial pressure of 0.1 to 50 mmHg as contacted with the high-purity low-silica type X zeolite adsorbent. When the partial pressure of carbon dioxide is too low, the adsorption efficiency is greatly reduced. In contrast, when the partial pressure of carbon dioxide is too high, the saturated adsorption is reached in too short time, making difficult to effect a conduct adsorption-desorption cycle and a continuous operation. Further, the too high partial pressure of carbon dioxide imposes a too high load to a pump, increases energy consumption and equipment cost.
The invention will now be described more specifically by the following working examples that by no means limit the scope of the invention.
Evaluation of zeolite and its shaped product was conducted by the following methods.
(1) Chemical Composition
A zeolite specimen is dissolved in a mixed solvent of nitric acid and hydrofluoric acid, and concentrations of metal ions are measured by an ICP (inductively coupled plasma) emission analyzer (xe2x80x9cOptima 3000xe2x80x9d available from Perkin-Elmer Co.). The concentration of a metal ion is expressed by an ion equivalent ratio. For example, the concentration of Li ion in a solution containing Li ion, Na ion and K ion is expressed by the ion equivalent ratio of Li ion represented by the formula Li/(Li+Na+K).
(2) Crystal Structure
Determination is conducted by an X ray diffraction apparatus (xe2x80x9cMXP-3xe2x80x9d available from MacScience Co.).
(3) Moisture Equilibrium Adsorption
A zeolite specimen is dried at a temperature of higher than 60xc2x0 C. is allowed to stand in a desiccator maintained at a temperature of 25xc2x0 C. and a relative humidity of 80% for at least 16 hours. Then ignition loss is measured by heating the specimen at a temperature of 900xc2x0 C. for 1 hour. The moisture equilibrium adsorption is calculated from the following equation.
xe2x80x83Moisture equilibrium adsorption (%)=[(X1xe2x88x92X2)/X2]xc3x97100
where X1 is weight as measured after standing in the desiccator but before the ignition, and X2 is weight as measured after the ignition.
(4) Adsorption Capacity
Adsorption capacity of carbon dioxide and adsorption capacity of nitrogen are measured by using adsorption measurement apparatuses xe2x80x9cBELSORP 28SAxe2x80x9d and xe2x80x9cBELSORP HPxe2x80x9d both available from BEL Japan Inc. at a temperature of 25xc2x0 C. or 40xc2x0 C. The specimen used is kept in vacuo (under a pressure of not higher than 1xc3x9710xe2x88x923 mmHg) at a temperature of 350xc2x0 C. for 2 hours before measurement of the adsorption capacity. The respective adsorption isotherms are approximated according to the Dual-Site Langmuir equation (Ind. Eng. Chem. Res. 1996, 35, 2477-2483).
Using the thus-obtained adsorption isotherm, (i) adsorption of carbon dioxide in a single-component system of carbon oxide under a pressure of 2 mmHg is calculated, and further, (ii)-(1) adsorption capacity of carbon dioxide and (ii)-(2) adsorption selectivity of carbon dioxide in a two-component system comprising carbon dioxide and nitrogen are calculated according to an ideal adsorbed solution theory (A.I.Ch.E.J., 1965, 11, 121-127). The adsorption selectivity of carbon dioxide is calculated by the following equation.
Adsorption selectivity of carbon dioxide (xe2x88x92)=(X1/PY1)/(X2/PY2)
wherein P:
total pressure of carbon dioxide and nitrogen(=6.239 atm.)
Y1: molar fraction of carbon dioxide (=0.00042)
X1: adsorption capacity of carbon dioxide at Y1
Y2: molar fraction of nitrogen (=0.99958)
X2: adsorption capacity of nitrogen at Y2
(5) Macro-Pore Volume and Average Pore Diameter
Using a mercury-intrusion type porosimeter (xe2x80x9cPoresizer-9310xe2x80x9d available from Micromeritics Co.), macro-pore volume and average pore diameter of an activated adsorbent is measured in a pressure range of 1 to 30,000 psi (for pore diameter of 60 xc3x85 (angstrom) to 200 xcexcm).
(6) Average Crush Strength
Following the testing method described in JIS-R-1608, average crush strength is measured as follows. An activated zeolite shaped product specimen in a particulate form in used. The crush strength varies depending upon the particle diameter of particulate specimen, and therefore, the specimen used herein is prepared by sieving the particulate specimen to pick up particles having a particle diameter of 1.4 to 1.7 mm. Using Kiya digital hardness tester (xe2x80x9cKHT-20Nxe2x80x9d available from Fujiwara Laboratories), a compressive load is applied to the sieved particulate specimen by pressing a stainless steel pressing plate with a diameter of 5 mm against the specimen at a constant rate of 1 mm/sec at room temperature and atmospheric pressure, and the maximum load (unit: kgf) which the specimen can withstand is measured. The measurement is conducted on 25 specimens and the crush strength is expressed by an average value of the 25 maximum loads.